Detection of the presence, position and/or speed of objects is common, especially in traffic, such as for example civil and military air traffic control, in parking assistance devices, navigation or undersea detection of objects, etc. Several detection systems are known, most of them using a “transmitter beam” and “antenna beam” to focus energy to “targets” and measure the return signals using the same or other “antenna” and a “receiver” to identify targets and measure their parameters. Such systems can use the sound wave properties in applications like “sonar” or electromagnetic wave signals as in the case of “radar” or similar technologies.
The coverage of such measurement system is limited by what is known as “line of sight”, i.e. the “antenna” needs to be located at a position with a view to the targets as unwanted objects or objects that are not of interest in between the antenna and the target can disturb detection or even prevent detection of the targets. Nevertheless, such unwanted objects are often unavoidable and unwanted detections of these objects can make it difficult to trace the targets of interest. Unwanted detections are called “clutter” and need to be rejected as they are obscuring the view of the observer.
Clutter rejection is a major and difficult issue in signal processing. For example in radar signal processing, the intensity of clutter from man-made constructions, forest, hills and mountains are usually several orders of magnitudes stronger than the returned signal of for instance a small aircraft. Very sophisticated techniques have been developed over a period of 50 years to minimize the impact of clutter to the detection of aircraft. Among these methods are the “MTI” (moving target indicator) technique and the “MTD” (moving target detector) technique and techniques using Doppler signal processing. Due to advances in computer technology, this has lead to a clutter level that is considered workable. On the side of hardware, better antennas with lower side lobes have been developed in order to reduce the illumination of the ground level where most of the returned clutter signals are received from. Unfortunately the level of side lobe reduction has practical limits induced by antenna size cost and environment.
A large number of solutions are focussed on filtering towards the properties of signals of interest. More specifically, the spectral content of a return signal is used to filter and reduce the clutter intensity. The latter can be based on the fact that moving targets exhibit a Doppler shift. However in doing so, signals representative of targets with low radial speed and arriving at the receiver at the same time as clutter signals are also rejected. As Doppler shift is proportional to the radial speed of the target, objects performing a tangential flight above a clutter area have a considerably reduced detection probability. Furthermore most radars have a range requirement that limits their Doppler filter capabilities. Air traffic Control (ATC) radars suffer from “blind speed” problems: if the sampling rate of the radar is not fast enough, under sampling occurs and the target can appear to be stationary to the radar even on a radial flight. In this case it is rejected by the clutter filter. With a radar performance according to the present state of art a clutter reduction of 50 dB is theoretically possible. However the main limitation of clutter filters based on the difference in spectral content of target versus clutter signals lies in the fact that also many clutter objects are not stable at all. Especially “Sea clutter” is a real challenge for any radar hitting the surface of water as the reflectivity is high and the object is often moving in erratic ways.
Another problem for accurate detection is the distance-dependency of clutter. In larger ranges where a radar or sonar is active, the clutter is usually low as Earth curvature limits the objects in view. In shorter ranges, the clutter signals rises very strong, as can be seen from the path loss formula presenting an inverse fourth order law with range. In fact most airport radars need to use additional measures in order not to saturate the receiver with too much clutter. If saturation takes place the MTI mechanisms fail to work and the target is lost for all speeds. Two methods are usually present to maintain the return signals in the dynamic range of the receiver.
1) The vertical antenna beam is designed asymmetrical to “roll-off” below a set elevation angle thereby reducing return signals from low elevation.
2) Most antennas have two or more beams pointing to a different elevation angles. For the first few hundred microsecond after transmission the “high-beam” (HB) is switched to the receiver. When the clutter returns are below saturation level the “low-beam” (LB) is used. To illustrate the effect, exemplary pictures FIG. 1a and FIG. 1b are provided, showing a zoom of the image available on the output of a radar receiver. FIG. 1a shows the signals when the low-beam LB is used whereby it can be seen that the view is completely cluttered with signals up to saturation level, completely masking a possible target of interest. The video on the HB is reducing the number of blips and brings the signals within the dynamic range of the receiver allowing further signal processing.
FIG. 2 illustrates a typical radar setup as used for tracking airport traffic. It shows a radar setup for a radar with dual beam, a low beam (LB) for tracking objects with lower elevation and a high beam (HB) for tracking objects with higher elevation. The transmitter is typically connected using a circulator to the LB since most of the power is required to illuminate targets at the largest distance who suffer from the highest path loss attenuation. After the emitter pulse has left the antenna, the receiver is first connected to the high beam (HB) as targets are expected at higher elevation angles and at short range clutter is too strong to point lower. After some elapsed and programmable time the receiver is switched to the lower beam as target elevation angle is lower on further range (corresponding with longer response time) and the antenna pointing angle must be lower to have enough gain on the target. A schematic representation of a processing system for processing signals according to this method is shown in FIG. 3.
Switching may be used as a coarse filter. This provides about 20 dB of clutter reduction at the expense of reducing the detection of low flying targets. The beam switch measure as such furthermore often is not enough to deal with saturation and usually “STC” (Sensitivity Time Control) is used to further attenuate signals that are too strong on short range. In order to allow such attenuation, memory maps controlled in azimuth and range are tuned on site in order to prevent the need for using all these attenuation mechanisms throughout the whole range to be scanned, as these attenuation mechanisms will also reduce the signals of objects of interest. As the intensity and position of clutter is affected by seasonal and weather effects the present clutter suppression mechanisms, that require labor intensive handcraft tuning, are set for worse case conditions and are often hindering proper aircraft detection unnecessary. This all results in the situation that only strong (large) targets can be observed in the areas of high clutter.
The signal of the receiver is fed to a processing module that can integrate the coherent responses of targets to receive processing gain while, if required, reducing the clutter by moving target indicator processing (MTI) and Doppler processing. These techniques are done in the frequency domain. A disadvantage of these techniques is that detections are missed when the targets have similar Doppler properties as clutter. False detections are created when clutter has different spectral content that can be filtered off. It thereby is a disadvantage that false detections will increase the detection threshold for a given area resulting in missing true targets of interest.
In view of the drawbacks of these clutter rejection filters, the probability of detection (Pd) of a typical dual frequency ATC PSR radar is rarely better than 90%. Most of the missed detection signals (10%) are due to the strong clutter signals.
It is to be noticed that remarkably in the past 20 years there has been shown little or no progress on this subject with the exception of the use of extremely expensive “phase array antennas”, where the antenna technique is used to a maximum to avoid illuminating clutter areas by tracking the horizon. Still this method requires the use of sensitivity time control to reduce the signals received through side lobes on short range and thus is masking the detection of low flying objects when the horizon screening angle is high as in mountain areas.